1. Field of the Invention
The present invention relates generally to underwater plow devices and more particularly to plow devices capable of making trenches for burying communications cables.
2. Description of Related Art
In order to protect undersea cables from hazards in the ocean environment, including fishing activity, shifting bottom conditions and even human intervention, undersea cables have been historically buried. The burial process usually requires that the cable be deployed by a surface ship feeding cable down to a self-propelled or towed sub-surface plow device or vehicle. Safe depths for cable burial depend on the hazards commonly encountered in that region of the ocean bottom. Typical burial depths are from one to six feet, where most commercial plows bury cable to a depth of one meter.
Most conventional sea plow designs prepare a trench by inserting a digging tool into the ocean bottom and dragging the plow behind a ship. This method is limited by the amount of force the ship can impart to the tool. As would be understood, great amounts of force are typically required for towing a plow at ocean depths because a passive (ship drawn) digging tool experiences increased plowing resistance due to the flow of pore water, i.e., water flow into the spaces or pores between the grains of sand. This occurs because the sand must shear in order for the digging tool to pass. In shearing, the grains must roll over one another. In so doing the spaces (pores) between the grains must expand. Water pressure in these pores must decrease in order to draw in water to keep the pores filled. The drag of this permeating pore water squeezes the grains together and in this way increases the frictional force required to shear the sand. Hence the pull force (referred to as drawbar) is increased. The faster and deeper the digging tool goes the greater the decrease in pore water pressure, and hence the greater the drawbar, up to a limiting speed where either cavitation (vacuum) occurs or where the grains begin to crush. The cavitation limit is a function of ocean depth. The grain crushing limit is a function of the sand properties.
Prior art plows for undersea burial of cable have typically consisted of either passive tools or some form of water assisted tool, each being pulled behind a ship. The purely passive plow tools tend to be forward or vertically raked with a forward raked toe to provide a downward force to maintain stability and keep the tool in the soil. These devices are limited to relatively low speeds in dense sandy soils because of pore water effects. Thus, for a ship with a maximum pulling capacity of 30,000 pounds, the maximum plowing speed in a very dense sand would be a small fraction of one knot, for example.
Water assisted plows are similar to the passive plow tools, however, they contain nozzles that introduce water directly in front of the digging tool. The idea is to provide water to feed the expanding pores of the dilating sand and thereby hopefully reduce drawbar. These tools achieve some drawbar reduction, but it is not clear whether this limited success is due to overcoming pore water effects or due to fluidizing sand and flushing it away. In either case, however, these tools are susceptible to clogging by becoming jammed with sand. Also, these tools have a potential startup problem. If the plow speed becomes too great, the solid sand wall can come in intimate contact with the blade, shutting off the jets. If this happens, drawbar increases, the tow cable stretches, and the solid sand wall becomes spring loaded against the leading edge of the blade. In this situation it is unlikely that the jet flow can be reestablished. Backing the tow ship in order to unload the tow cable is an undesirable option.
Because of the towing forces required to operate the undersea plowing devices, large, specialized cable ships capable of providing large towing forces are still necessary. However, these cable ships exist in limited number. Accordingly, there is a need in the art of undersea trenching to provide a plow device that reduces the towing forces presently required. This would increase the number of ships which are capable of laying cable, and reduce the expense and delay encountered with the use of traditional plows.